Optically variable pigments have been described in the patent literature since the 1960s. Hanke in U.S. Pat. No. 3,438,796 describes the pigment as being xe2x80x9cthin, adherent, translucent, light transmitting films or layers of metallic aluminum, each separated by a thin, translucent film of silica, which are successively deposited under controlled conditions in controlled, selective thicknesses on central aluminum film or substratexe2x80x9d. These materials are recognized as providing unique color travel and decorative optical color effects.
The recent approaches to optically variable pigments have generally adopted one of two techniques. In the first, a stack of layers is provided on a temporary substrate which is often a flexible web. The layers are generally made up of aluminum, chromium, magnesium fluoride and silicon dioxide. The stack of film is separated from the substrate and subdivided into appropriately dimensioned flakes. The pigments are produced by physical techniques such as physical vapor deposition onto the substrate, separation from the substrate and subsequent comminution or by other. deposition techniques (plasma, sputtering etc.), subsequent deflaking of the decomposition product, etc. In the pigments obtained in this way, the central layer and all other layers in the stack are not completely enclosed by the other layers. The layered structure is visible at the faces formed by the process of comminution.
In the other approach, a platelet shaped opaque metallic substrate is coated or encapsulated with successive layers of selectively absorbing metal oxides and non-selectively absorbing layers of carbon, metal sulfide, metal and/or metal oxide. To obtain satisfactory materials using this approach, the layers are applied by multiple techniques such as chemical vapor deposition and/or sol-gel processes. A major shortcoming of this is that traditional metal flakes usually have structural integrity problems, hydrogen outgassing problems and other pyrophoric concerns.
The prior art approaches suffer from additional disadvantages. For instance, certain metals or metal flake such as chromium, aluminum, copper, brass and bronze may have perceived health and environmental impacts associated with their use. The minimization of their use in optical effect materials should be advantageous due to their perceived impact.
The present invention provides a color effect material (CEM) comprising a platelet-shaped substrate encapsulated with:(a) a highly reflective first layer to light directed thereon and being selected from the group consisting of silicon, aluminum, titanium nitride and mixtures thereof; and (b) a second layer encapsulating the first layer and providing a variable pathlength for light dependent on the angle of incidence of light impinging thereon in accordance with Snell""s Law; and (c) a selectively transparent third layer to light directed thereon. CEM""s provide optically variable pigment attributes as well as additional non-decorative effects such as conductivity, EMI/RFI shielding and/or desirable tactile properties.
It is an object of the present invention to provide novel CEM""s which can also be prepared in a reliable and reproducible manner. This object is achieved by a CEM comprising a platelet-shaped substrate encapsulated with:(a) a highly reflective first layer to light directed thereon; and (b) a second layer encapsulating the first layer in which the second layer consists of a low index of refraction material, typically a refractive index from 1.3 to 2.5 and more specifically between 1.4 and 2.0 that provides a variable path length for light dependent on the angle of incidence of light impinging thereon; and (c) a selective transparent third layer to light directed thereon. The degree of reflectivity for the first encapsulating layer should be from 100% to 5% reflectivity, whereas the selective transparency of the third encapsulating layer should be from 5% to 95% transmission. More specifically, one would prefer to have 50-100% reflectivity and 50-95% transparency for the first and third encapsulating layers, respectively. The degree of reflectivity and transparency for different layers can be determined by measuring a variety of methods such as ASTM method E1347-97, E1348-90 (1996) or F1252-89 (1996). The substrate can be mica, aluminum oxide, bismuth oxychloride, boron nitride, glass flake, iron oxide-coated mica (ICM), iron oxide coated glass flake, silicon dioxide and titanium dioxide-coated mica, titanium dioxide coated glass flake (TCM), variations of the above-mentioned substrate, or any encapsulatable smooth platelet. The first and third layers can be the same or different materials, i.e., aluminum, silicon, titanium nitride or mixtures thereof.
Preferably, all the layers are deposited onto any particulate substrate by chemical vapor deposition (CVD) from an appropriate precursor (The Chemistry of Metal CVD, edited by Toivo T. Kodas and Mark J. Hampden-Smith; VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1994, ISBN 3-527-29071-0). In this case, there is a particular advantage over the prior art in that the products of the present invention can be produced in a reactor without having to be removed during stages of the preparation for additional processing. The CVD reactor can handle the coating of all the layers, calcining, and exterior treatments without the powder substrate being removed from the reactor until completion. No additional processing or communition of the material is necessary for obtaining the desired final particle size and/or effect product.
The products of the present invention are useful in automotive, cosmetic, industrial or any other application where metal flake, pearlescent pigments, or optically variable pigments are traditionally used.
The size of the platelet-shaped substrate is not critical per se and can be adapted to the particular use. In general, the particles have average largest diameters of about 2-250 xcexcm, in particular 5-100 xcexcm. Their specific free surface area (BET) is in general from 0.1 to 25 m2/g.
The CEM""s of the invention are notable for multiple encapsulation of the platelet-shaped substrate.
The first encapsulating layer which is made of the group consisting of silicon, aluminum and titanium nitride, or their mixtures is highly reflective to light directed thereon.
The thickness of the first encapsulating layer is not critical so long as it is sufficient to make the layer highly reflective. If desirable, the thickness of the first layer can be varied to allow for selective transmission of light. When silicon is selected as the component of the first layer, the thickness must be at least about 20-100 nm, preferably from about 30 to 75 nm. However, when aluminum is used, the thickness must be at least about 10-90 nm, preferably from about 20 to 65 nm. The thickness of titanium nitride as the first layer must be at least about 15-95 nm, preferably from about 15 to 60 nm. A thickness outside of the above-mentioned ranges can also be utilized depending on the desired effect. The quantity of the coating will be directly related to the surface area of the particular substrate being utilized.
The second encapsulating layer must provide a variable pathlength for light dependent on the angle of incidence of light impinging thereon and therefore, any low index of refraction material that is visibly transparent may be utilized. Preferably, the second layer is selected from the group consisting of silicon dioxide (SiO2), suboxides of silicon dioxide (SiO0.25 to SiO1.95) or magnesium fluoride.
The thickness of the second encapsulating layer varies depending on the degree of color travel desired. In addition, the second encapsulating layer will have a variable thickness depending on a variety of factors, especially refractive index. Materials having a refractive index around 1.5 tend to require a film thickness of a few hundred nanometers for generation of unique extensive color travel. For instance, a second layer has a preferable thickness of about 75 to 500 nm for silicon dioxide and for magnesium fluoride. A thickness below this range will result in limited color travel, which may be desirable for specific applications.
In one embodiment, the second layer is encapsulated by a selectively-transparent third layer that allows for partial reflection of light directed thereon. Preferably, the third layer is selected from the group consisting of silicon, iron oxide, chromium oxide, titanium dioxide, titanium nitride, a mixed metal oxide, aluminum and mixtures thereof.
Of course, the third layer can also contribute to the interference of the pigment. Its thickness can vary but must always allow for partial transparency. For instance, a third layer has a preferable thickness of about 5 to 20 nm for silicon; about 2 to 15 for aluminum; about 1-15 nm for titanium nitride; about 10 to 60 for iron oxide; about 10 to 60 for chromium oxide; about 10-100 nm for titanium dioxide and about 5 to 100 nm for a mixed metal oxide, or mixtures thereof.
All the layers of the CEM of the invention are altogether notable for a uniform, homogeneous, film-like structure that results from the manner of preparation according to the invention.
In the novel process for preparing the encapsulated platelet-like substrates, the individual coating steps are each effected by reaction or thermal decomposition of suitable starting compounds in the presence of the substrate particles to be coated. For instance, silicon can be deposited from silane (SiH4), alkyl and aryl substituted silanes (Rxe2x80x94SiH3), and halogen substituted silanes such as trichlorosilane, dichlorosilane and chlorosilane. Some additional precursors to forming silicon include 1,2-disilylethane, disilylmethane, t-butylsilane, bis(t-butyl)silane and phenylsilane. Analogs of these mentioned compounds can be used as well. Numerous aluminum precursors exist such as the dialkyl aluminum hydrides, trialkyl aluminums and Lewis base stabilized alanes (L-AlH3). Some specific examples include dimethyl aluminum hydride, tri-isobutyl aluminum, trimethylamine:alane, ethyldimethylamine:alane and N-methylpyrrolidine:alane; silicon dioxide from a compound selected from the group consisting of silicon tetraalkoxides such as tetraethoxysilane, di-t-butoxydiacetoxysilane and silicon tetrachloride; titanium nitride from titanium tetrachloride and the dialkyl amides of titanium such as tetrakis(dimethyl-amido) titanium (TDMAT) and tetrakis(diethyl-amido) titanium (TDEAT); iron oxide from an iron carbonyl, iron chloride and iron sulfates; and chromium oxide from a chromium chloride, chromium sulfates and chromium carbonyl.
The coating of each of the layers is preferably carried out in a heatable fluidized bed reactor, as described for example in EP-A-33 457 or DE-A-38 13 335, where the uncoated or already singly or doubly coated substrate particles are initially fluidized in a fluidizing gas and heated to the temperature sufficient for decomposing the particular precursor compound. For instance, the temperature sufficient for reacting or decomposing silicon precursors for silicon deposition is about 400 to 750xc2x0C.; for reacting or decomposing aluminum precursors for aluminum deposition, about 200 to 400xc2x0C.; for reacting or decomposing titanium nitride precursors for titanium nitride deposition, about 300 to 800xc2x0C.; for reacting or decomposing iron oxide precursors for iron oxide, about 70 to 200xc2x0C.; for reacting or decomposing titanium dioxide precursors for titanium dioxide deposition, about 150 to 350xc2x0C.; and for reacting or decomposing silicon dioxide precursors for silicon dioxide deposition, about 100 to 550xc2x0 C. All the reaction or decomposition temperatures are precursor and reactor specific. The vaporized precursor compounds and any gases required for achieving the reaction or decomposition are then introduced via separate nozzles.
Each of the layers is deposited by reacting or decomposing precursor compounds as indicated hereinabove, preferably alkyl and aryl substituted silanes, dialkylaluminum hydrides, trialkylaluminums, Lewis base stabilized alanes and chlorides, tetradialkyl amidotitanium, carbonyls, and alkoxides, such as phenoxides and benzyl alkoxides but also aliphatic alkoxides, in particular ethoxides. A variety of inert carrier gas systems can be utilized for the silicon and aluminum precursors such as nitrogen, helium and argon. Pure oxygen, hydrogen and/or air can be utilized to react and/or decompose these materials depending on the desire for an oxidizing or reducing atmosphere. Similar carrier gases would be utilized for the titanium nitride precursors, however, ammonia gas (NH3) is typically utilized as one of the gases for reacting or decomposing such precursors to form titanium nitride. Iron carbonyl is transported to the reaction zone in an inert carrier gas such as nitrogen or helium and in some instances may contain some, or exclusively be, carbon monoxide gas. The carbonyl is reacted or decomposed by addition of oxygen and/or air at the reaction zone resulting in the deposition of iron oxide on the particulate surface whereas the alkoxides are hydrolyzed by air and/or water vapor. The oxygen or water vapor must be supplied in an amount which is at least that required stoichiometrically for forming the desired oxide or, in the case of the carbonyls, carbon dioxide, but it is also possible to use a small excess. The fluidizing gas used in this coating operation can be not only nitrogen but also air depending on the reactivity of the precursor reagents.
All the coating operations are advantageously carried out by vaporizing the precursor compounds in a vaporizer upstream of the reactor and transporting the resulting vapors into the reactor by means of a carrier gas, in general air, nitrogen, hydrogen/nitrogen mixtures, or other gases, depending on the reaction conditions required.
To obtain homogeneous layers that encapsulate the substrate uniformly and completely, the gas quantity of the precursor compound should in general preferably not be more than 5% by volume, more preferably not more than 2% by volume, of the total quantity of gas in the reactor. Depending on the deposition efficiency, it may be necessary to greatly increase the concentration of the precursor compound in the total quantity of gas.
As indicated hereinabove, the product need not be isolated between different coating operations; on the contrary, a further coating operation can advantageously be carried out immediately after a previous coating operation in the same reactor, if desired after a small reduction in temperature and if necessary after the substitution of a different fluidizing gas, or precursor compound carrier gas.
If the applied layer is a metal or a low metal oxide, which is to form the outer layer of the particulate, it is advantageous to mix air into the fluidizing gas during cooling in order that the layer surface may be passivated by oxidation. The cooled product is then discharged in the usual manner.
The process of the invention makes it possible to prepare multiply coated CEM""s in a simple manner. The thickness of the layers produced can be set specifically and reproducibly to the desired value via the coating time, the concentration of precursor compound used and the ratio of initially charged particles to added precursor compound. The CEM""s obtained in this way are notable for the high quality of the coating, i.e. for homogeneous, uniform layers encapsulating the substrate particles in a film fashion.
The color effect materials (CEM""s) of the invention are advantageous for many purposes, such as the coloring of paints, printing inks, plastics, glasses, ceramic products and decorative cosmetic preparations. Their special functional properties make them suitable for many other purposes. The CEM""s, for example, could be used in electrically conductive or electromagnetically screening plastics, paints or coatings or in conductive polymers. The conductive functionality of the CEM""s makes them of great utility for powder coating applications.
The above mentioned compositions in which the compositions of this invention are useful are well known to those of ordinary skill in the art. Examples include printing inks, nail enamels, lacquers, thermoplastic and thermosetting materials, natural resins and synthetic resins, polystyrene and its mixed polymers, polyolefins, in particular polyethylene and polypropylene, polyacrylic compounds, polyvinyl compounds, for example polyvinyl chloride and polyvinyl acetate, polyesters and rubber, and also filaments made of viscose and cellulose ethers, cellulose esters, polyamides, polyurethanes, polyesters, for example polyglycol terephthalates, and polyacrylonitrile.
Due to its good heat resistance, the pigment is particularly suitable for the pigmenting of plastics in the mass, such as, for example, of polystyrene and its mixed polymers, polyolefins, in particular polyethylene and polypropylene and the corresponding mixed polymers, polyvinyl chloride and polyesters in particular polyethylene glycol terephthalate and polybutylene terephthalate and the corresponding mixed condensation products based on polyesters.
For a well rounded introduction to a variety of pigment applications, see Temple C. Patton, editor, The Pigment Handbook, volume II, Applications and Markets, John Wiley and Sons, New York (1973). In addition, see for example, with regard to ink: R. H. Leach, editor, The Printing Ink Manual, Fourth Edition, Van Nostrand Reinhold (International) Co. Ltd., London (1988), particularly pages 282-591; with regard to paints: C. H. Hare, Protective Coatings, Technology Publishing Co., Pittsburg (1994), particularly pages 63-288. The foregoing references are hereby incorporated by reference herein for their teachings of ink, cosmetic, paint and plastic compositions, formulations and vehicles in which the compositions of this invention may be used, however not limited to, including amounts of colorants. For example, the pigment may be used at a level of 10 to 15% in an offset lithographic ink, with the remainder being a vehicle containing gelled and ungelled hydrocarbon resins, alkyd resins, wax compounds and aliphatic solvent. The pigment may also be used, for example, at a level of 1 to 10% in an automotive paint formulation along with other pigments which may include titanium dioxide, acrylic latices, coalescing agents, water or solvents. The pigment may also be used, for example, at a level of 20 to 30% in a plastic color concentrate in polyethylene.